Apparatus for coating articles with a dry powdered material

ABSTRACT

An apparatus are disclosed for coating an article of manufacture to be varnished by coating it with a powdered resin and then heat-cured. A localized powder-cloud is formed internally of a tunnel, the tunnel overlying a tub in which the powdered resin is contained. Means are provided for forming a fluidized bed with the powder particles and means are also provided to provide a temporary adhesion of the powder to the articles (e.g. a glue or a system of electrostatic charges) prior to the final anchoring obtained by curing.

United States Patent Venturi 1 1 Sept. 12, 1972 [54] APPARATUS FORCOATING ARTICLES 3,233,584 2/1966 Angstadt ..118/429 WITH A DRY POWDEREDMATERIAL 3,241,520 3/1966 Wurster et a1. ..118/62 Inventor: EnricoVenturi, Via Lorenteggio,

39, Milan, Italy Filed: May 22, 1970 Appl. No.: 39,909

US. Cl ..118/629, 117/17, 118/DIG. 5, 1 18/621 Int. Cl ..B05c 5/02 Fieldof Search ..118/621, 623, 624, 629, 630, 118/429, 400.5, DIG. 5; 117/17,21

References Cited UNITED STATES PATENTS 10 1959 Weiner ..1 18/400512/1962 Sigvardsson et a1. 18/629 4/1966 Barfordet a1 ...11s/400.5

Primary ExaminerMorris Kaplan Assistant Examiner-Leo MillsteinAttorney-Diller, Brown, Ramik & Holt [57] ABSTRACT An apparatus aredisclosed for coating an article of manufacture to be varnished bycoating it with a powdered resin and then heat-cured. A localizedpowdercloud is formed internally of a tunnel, the tunnel overlying a tubin which the powdered resin is contained. Means are provided for forminga fluidized bed with the powder particles and means are also provided toprovide a temporary adhesion of the powder to the articles (e.g. a glueor a system of electrostatic charges) prior to the final anchoringobtained by curing.

1 Claim, 6 Drawing Figures APPARATUS FOR COATING ARTICLES WITH A DRYPOWDERED MATERIAL This invention relates to an apparatus for applying,in a continuous and automatic fashion, particles of a dry material to anarticle of manufacture. More particularly, the invention is concernedwith finishing operations whereby the outer surfaces of articles ofmanufacture are coated with a continuous film of a plastics material.

The method for varnishing, plastic-coating and anyhow coating articlesof manufacture by applying dry powders thereto are known long since andcommercially applied on a large scale.

Basically, these methods tend to deposit on the surface of an article ofmanufacture the coating material (which is generally a synthetic resinto which dyeing pigments may be added if so desired) in the form of afinely divideddry powder and, in a subsequent stage, to transform'thepowdery film or layer as deposited on the surfacesof'the articleconcerned, into a continuous, homogeneous and firmly adherent coating.As a rule, said transformation is brought about by melting theparticles, with a possible concurrent polymerization thereof, saidresults being obtained by heat treatment in an oven, in very much thesame way as is practised with the conventional varnishes dissolved in asolvent.

The techniques adopted heretofore for the industrial achievement ofthese methods can be substantially grouped into two categories, viz.:

a. Dipping in a fluidized bed b. Spraying In the former instance, a tubopen at the top is provided, whose bottom wall is a porous slab. The tubis charged with a certain amount of coating material, in the form of avery fine powder and in a dry condition, which settles on the bottomwall. Through the porous slab an air stream is sent, at not too high aspeed, which lifts the powdered particles which thus become an airbornemass: thus the apparent volume of the mass is increased and the massitself takes the appearance and the properties of a thick fluid keptunder not too brisk a boil. If now the article of manufacture, which hadbeen previously wetted by glue or, as an alternative, preheated to themelting point temperature of the powder, the powdery particles whichsurround the article concerned are deposited thereon in the form of apowdery layer stuck to the surfaces of the article. The article is thenheat treated in a curing oven as outlined above.

This method is impaired by a number of shortcomings, the most serious ofwhich is that the article to be coated should be previously wetted by aglue or preheated to a high temperature. Another drawback stands in thelimitations which are encountered in practice when attempting to realizefluidized beds having a horizontal projected surface exceeding 100-150square decimeters. Another significant defect is that the entire workingcycle is discontinuous and, at any rate, it requires working times whichconflict with the necessities of an industrial mass-production.

An improvement over the method aforementioned consisted in exploitingelectrostatic attraction forces to cause the layer of powdered materialinitially to stick to the workpieces.

To this end, a cathode grid has been inserted in the fluidized bed, bycausing the workpieces to act as anodes and establishing between saidanode and cathode an appropriately high potential differential.

Even so, however, the defects aforementioned of discontinuity inprocessing and long duration in time of same are not overcome, the samebeing also true of the defects inherent in the intrinsic limitations ofthe fluidized bed.

Methods and apparatus of the second category, i.e. (b) as mentionedabove, essentially consist in using a spraying gun to which an air orgas-stream is fed so that the dry particles of the coating compositionare suspended and hurled against the surfaces of the workpiece to becoated. Prior to emerging from the muzzle of the spraying gun theparticles are caused to pass through an electrically conductiveenvironment with a potential differential of a few kilovolts withrespect to the article to be coated, which has an opposite polarity. Thepowder particles, which now possess an electrostatic charge, are drawnby the workpiece and are deposited on the surfaces thereof to form afilm or layer which is then converted into a continuous film by theusual curing operations.

As is well known, this method is predominantly manual one and thus isbound the employment of operators to whose skill the task is entrustedto estimate the thickness uniformity of the layer as deposited onto theseveral faces of the article to be coated.

In addition, another drawback which should not be overlooked is the lossof the material which is scattered through the environment where thespraying operations are carried out.

Mechanization and automation of the use of spraying guns, both singleand multiple, and also both fixed and reciprocated, become extremelyawkward in the case of articles of manufacture having an intricate shapeand/or being not mass-produced, on account of the variations of thedistances between the spraying nozzle(s) and the several points of theworkpiece. 1

In the case of articles of manufacture having an extremely simple anduniform configuration, such as pipes, bands and the like, continuouscoating apparatus have been embodied in which the article concerned iscaused continuously to pass along the axis of a substantiallycylindrical chamber having fixed nozzles circumferentially affixedthereto which are adapted, both as to their number and positions, tocause a spray of airfluidized powder, which has been electrostaticallycharged with conventional methods, to impinge on the workpieceseverywhere.

The particles of the material which are not deposited on the workpiecefall down on the bottom of the chamber and are recovered therefrom andcarried out towards the exterior, to be recycled after appropriate andessential filtration, classification and admixture with fresh powder,for an amount which generally is not less than 50 percent. The principaldefects of this method are: (1) the limitation to articles having aconstant cross-section; (2) the intricacy of the ancilliary apparatusfor recovering and recycling the fallout.

This invention overcomes the problems and defects briefly outlined aboveand, to this end, it provides an apparatus of the kind comprising a tubwherein a fluidized bed on particles of a dry material is generated byintroducing through the bottom a fluidizing gas, is characterized inthat it comprises a tunnel of which the More particularly, the apparatusaccording to the' present invention provides, for creating the abovecited particle cloud, the use of at least a blower or helical passed sothat each point of the article stays in the area occupied by thecloud'for at least 2 seconds. The adhesion forcejbetween the particlesand the articles is generated with known conventional means as alreadyoutlined above. More particularly, and preferably said force is'of anelectrostatic nature and is impressed onto the particles, byarranging,downstream of the blower orpropeller, an electrodic corona-dischargedevice, well known in the art, whereas the article to be coated isconnected to the other pole of the generator which feeds the deviceaforesaid, or anyhow with a source of electricity adapted to impress tothe article to be coated a charge having a sign opposite to that of theparticles which make up the fluidized bed. As an alternative, in orderto electrify the particles, air, which has been previously ionizedandadmixed with the proportion of particle suspension, can be used.

The tunnel, in practice, is divided into three portions, of which thecentral one is occupied by the cloud of particles and the side portionsare a mere extension of the central portion for recovering the particleswhich fall out of the cloud. As a matter of fact, the proportion of thesuspension which is sent to the central portion by the blower or thepropeller is composed by particles which, when they are not deposited onthe article and thus carried away thereby, fall freely into theunderlying tub.

It is obvious that the time of stay of each cross-sec tion of thearticle to be coated in the volume occupied by the cloud, as aforesaid,once that a working speed or advance speed of the article to be coatedalong the tunnel has been established, defines the length of the centralportion: the side portions or recovery portions in turn will have, as anaverage, the same length as the central portion.

The height of the tunnel, more particularly of the empty space above thefluid bed, should not be less than twice the height of the cross-sectionof the article,

that is, its maximum vertical dimension in the position taken during.conveyance, whereas all the points of the tunnel and thus it has thesame dimensions as the tunnel, in plan view, whereas the height of theedge of the tub, as measured starting from the porous diaphragm, shouldbe at least twice the height of the so-called fixed bed, that is, thethickness of the layer of dry powdered material when no fluidization gasis fed. By so doing, a free expansion is ensured even in the limitingcase where the volume ofthe fluidized bed is doubled.

The aforementioned cloud of particles of dry material is formed byresorting to either of two methods,

namely: V

a. employing one or more helical propellers with the drawing pipeimmersed in the fluid bed so as to draw the material at about one halfof the tub height. The drawing area is defined by vertical walls,possibly cylindrical, which separate the drawing space from theremaining part of the fluid bed without however cutting the freecommunication and the circulation of the particle suspension whilepreventing undesirable whirling motion from being impressed to the fluidbed.

. using one or more centrifugal blowers mounted on the outerlongitudinal sidewalls of the tub, whose drawing mouthisopened at aboutone half of the tub height into the'fluidized' bed. The delivery'side ofthe blower, or of each blower, opens, through appropriate manifolds,into the immediately overlying tunnel wall so as to originate the clouddescribed above. I It is obvious that both the number and thearrangement of the helical propellers as well as of the centrifugalblowers are a function of the size of the tunnel central area whichshall be occupied by the suspension. Their rate of flow should beselected on the basis of the following factors:

1. amount of particles which stick to the article to be coated, as anaverage. 2. volume of the cloud of suspended particles 3. concentrationof solids'required by the suspension. 4. concentration of solids in thefluidized bed 5. fallout velocity of the particles. Usually andpreferably, the concentration of solids in the fluidized bed is 300-600grams per liter, whereas the amount of particles taken up by the articleis 40-120 grams per square meter. The fallout velocity of the particles,according to the specific gravity and the granule size of the powder,usually varies between a fraction of one centimeter per second and 35cms. per second. On taking into account, lastly, that the density orconcentration of solids in the cloud should be at least 5 grams perliter, and preferably comprised between 7 and 15 g/liter, it is easy, onthe basis of the other parameters of the tunnel, to calculate the numberand the rate of flow of the lifting means for the proportion ofparticles of the fluidized bed. According to a first embodiment of theinvention, the lifting means are arranged on a side of the tub only andgenerate a whirling cloud which tends to fall on the opposite side.

According to an alternative embodiment, the lifting means are arrangedon both sides of the tub so that an enhanced turbulence is obtained forthe cloud along with a more frequent renewal of fresh suspension.

Inasmuch as almost all of the resins used to coat articles ofmanufacture tends to be charged with negative electricity, the articleshould be placed at the positive leg of the generator. In practice, forreasons of safety and to avoid hazards for the workers it is preferredthat both the article to be coated and the relevant pole of thegenerator be grounded as well as the conveyor. To this end, moreover,the tunnel walls will be coated on both faces with a plastic materialhaving a high insulating power. Preferably, these walls are made of aresin with a metal insert, if necessary. By so doing it is also avoidedthat the powder of the suspension formingthe cloud be deposited on thewalls.

The invention will be now described in connection with a fewpreferred'embodiments, without any limitation and having reference tothe accompanying drawings, wherein:

FIG. 1 is a cross-sectional view of the device according to theinvention, taken along the line lI of FIG. 2;

FIG. 2 is a plan view partly in section of the device of FIG. 1;

FIG. 3 is a view similar to FIG. 1 of an alternative embodiment of thedevice;

, FIG. 4 is a view similar to FIG. 3, of another modification.

FIGS. 5 and 6 are views, similar to FIGS. 1 and 2, respectively, of afurther modification.

Having now reference, at the outset, to FIGS. 1 and 2, they show adevice comprising a tub 10 having a bottom wall 11 and adouble bottomformed by a porous diaphragm 12 above which a fluidized bed 13 is formedby the agency of air fed between the bottom 11 and the porous diaphragm12 via the ducts 14. Above the tub 10 a continuous tunnel or covering 15is provided, which is affixed with appropriate means (not shown) alongthe lower edge 16, to the upper edgel7 of the tub l0. Along the summitline of the tunnel 15 there is an aerial conveyor formed by a boxlikecasing 18, equipped with a rail 19 on which wheels 20 ofhooks 21 canslide, to which the articles 22, to be coated with particles of drypowdered material forming the fluidized bed 13 are hung. The hooks 21,as is usual in the technique of aerial conveyors, will be set atappropriate intervals along a continuous chain (not shown) which ispowdered by conventional motive means (also not shown).

It is important to notice that the contact between the article 22 andthe hook 21 should always be such as to ensure a thorough conduction ofelectricity therebetween. Along'one sideof the tub 10 and externally, bymeans of appropriate supporting members 23, electric motors 24 aremounted (in the number of three in the case of FIGS. 1 and 2), whoseshaft 25 carries, at the end placed in the interior of the tub 10 ablade propeller, or helix 26., situated approximately midway of theheight of the tub 10. Of course, appropriate sealing gaskets (not shown)are located where the shaft 25 enters the tub 10.

A partition or diaphragm 27 is mounted in a suspended position in thefluidized bed 13 by the agenc'y of suitable means, as exemplified by therods 28, so as to delimit, in part, an environmental area 29 in theinterior of the fluidized bed 13, the width of this area being equal (orso) to the working diameter of the propeller 26 and the length beingsuch as to encompass all of the propellers 26. In correspondence witheach propeller, and above the same, conventional electric devices areprovided for delivering corona discharges,

as diagrammatically indicated at the electrodes 30 and 31, respectively,of which the electrodes 30 are fed through a common commutator 32connected to the negative pole 33 of a generator of DC voltage (notshown) whereas the electrodes 31 are grounded, as is usual. The positivepole 34 of the generator is grounded and connected through the lead 35to the boxlike casing 18 of the overhead conveyor. In this connection itshould be noted that, to prevent powder from entering the boxlike casing18, appropriate brushes 37 are provided, mounted along the whole tunnel15.

It should be observed in FIG. 2 that the tunnel 15 is ideally dividedinto three portions (as shown in dotted lines at 36) the central one ofwhich, 38, is completely filled by the cloud as generated by lifting theproportion of particle suspension from the fluidized bed 13, whereas theside portions 39 are intended only as extensions of the central portionfor recovering the fallen particles. In the embodiment shown in FIG. 3(wherein parts equal to those of FIGS. 1 and 2 are indicated by equalreference numerals), for lifting the proportion of particle suspension acentrifugal blower is provided which has a hood 40 mounted externally ofthe tub 10. An external motor 41 actuates via the'shaft 42 a vaned rotor43. The suction end 44 of the centrifugal blower opens directly throughthe tub wall at a height which is preferably in the order of one halfapproximately of the tub height whereas the delivery side of the blowerconsists of a guiding duct 45 which opens freely in the tunnel l5 and isappropriately radiussed to the curvature of the tunnel wall so as toform a swirling cloud around the article 22. In the manifold 45 isinserted, as is the case in FIG. 1, a device capable of imparting acoronaeffect electrostatic charge to the particles of dry powderedmaterial.

The embodiment shown in FIG. 4 differs from the one shown in FIG. 3inthat the suction end of the centrifugal blower is connected to a duct46 for introducing ionized air as supplied by a device indicatedgenerally at 47 and in which, in a manner which is well known in theart,'air is fed, which is ionized, for example, by electric discharges.The ionized air which enters through the suction end 44 is admixed tothe proportion of fluidized bed which has been drawn up and gives riseto the electrification of the resin particles. Of course, the boxlikecasing 18 of the overhead conveyor is appropriately grounded.

In the embodiment shown in FIGS. 5 and 6, the same type of propeller asshown in FIGS. 1 and 2 is illustrated, the only difference being thatthe diaphragm 27 consists of individual cylindrical diaphragms whichencompass areas 51 where the desired proportions of particle suspensionare lifted from the fluidized bed.

The basic feature of the claimed method remains now to be emphasized,that is, the feature of generating a cloud of particles through whichthe article to be coated is caused to pass at an appropriate speed andthe particles of the cloud fall into the fluidized bed again and arerecycled without any intermediate recovery or treatment of the particlesinvolved.

Practical experiments have been conducted with an apparatus having thefollowing specifications.

A fluid bed has been originated in the tub, starting from a fixed bedheight of 40 cms. The tunnel had a height of 70 cms. as measured interms of height of the chamber above the tub. The volume of thesuspended cloud was 600 cubic decimeters. The tests were carried outwith an epoxy resin having an apparent specific gravity of from 500 to600 grams per liter, using test strips formed by carbon stell sheetshaving a thickness of 0.8 mm. and a size of 20 by 30 cms., bent midwayof the longer side and having an overall exposed surface of 12 squaredecimeters.

The fluidized bed had a density of 350-430 g/liter whereas the suspendedcloud contained a weight of powder of from 4,500 to 6,000 grams. To liftthe cloud, a centrifugal blower has been used, whose rate of flow wasadjustable at the intake side and which had a rate of flow of from 1.6to 3 liters per second, corresponding to a lift of 800 to 1,250 grams ofpowder per second.

Variations in thickness between the several faces of the test strip didnot exceed 10 percent.

What is claimed is:

1. An apparatus for coating articles of manufacture with a dry powderedmaterial, of the kind comprising a tub in which a fluidized bed isgenerated with particles of a dry material by introduction of fluidizingair through the tub bottom wall, characterized in that it comprises atunnel of which the tub is the bottom wall, conveying means adapted tocause articles of manufacture to be passed in succession along thetunnel axis,

The electrostatic charge to the particles has been immeans situated insaid tub and adapted to send a proportion of said suspension ofparticles on the portion of the tunnel which overlies the tub so as toform an atmosphere of particles which extends along about one third ofthe tunnel length centrally with respect to the ends thereof, means forgenerating an adhesion force between the particles and the articles,said lifting means consisting of at least one centrifugal blower mountedexternally of the tub, the suction end of the blower being openedthrough'the tub sideabout halfwalls of the conveying duct beingradiussed to those of thetunnel.

g; UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. 3 9029 Dated September 12, I972 Inventor(s) ENRICO VENTURI It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

On the abstract page, read the following: Claims rpriority, applicationItaly,- May 24, 1 969, 17,291 A/69 Signed and sealed this Zhth day ofApril 1973.

( L) Attest: v

EDWARD M.FLETCHER,JR. t ROBERT GOTTSCHALK Atte sting OfficerCommissioner of Patents

